Unless you export the mask, and flip it in Photoshop, then re-import it back.
Steps:
Click your mask and export it:

Load your file into your image editing program and make your changes.
Save, then drag into your shelf, or File->Import Resource and make sure you select TEXTURE:

Now to use the new mask you need to select your fill layer or whatever you're using and add Bitmap Mask:

Then type in your mask name, and it will work!

If there is another way to do it in Painter I'm not aware of it, but this is what I personally do.
I hope this helps!